IT’S RAINING SPIDERS

Finally, we get to the final post of spider behavior month (ok, so maybe it took THREE months, whatever): the post on spider social behavior!

Social behavior, you say? In MY spiders?

Yes! Indeed there is, though spiders are known (for good reason) for being antisocial loners… sometimes even cannibalistic antisocial loners. One hypothesis for the evolution of so many frantic and flashy spider mating displays is, in fact, that the poor males are just trying to convince the female to let them pass on a bit of sperm before they get chewed into a pulp.

Honestly, you might think that spiders are so successful as solitary hunters that they would have no reason to ever try to team up- and you’d mostly be right. Of the 40,000+ species of spiders that we know of, only around 80 or so are known to some of their lives living together in large groups.

So what is different about these chosen few? What does it mean to be a social spider, and what evolutionary pressures lead to this striking change in behavior?

Take my hand, and I will lead you into a magical forest, where the trees look just like cotton candy, and when a strong enough wind blows, a rain of spiders falls upon your head.

The itsy-bitsy spider crawled up the water spout, down came the rain…

Before we talk about huge colonies of spiders, let’s talk about the more modest social behaviors found within Araneae. The most basic of these, of course, is the social behavior required to communicate intentions to do the do. For even the most antisocial, aggressive animal needs to to be able to survive this particular encounter with their own kind. (Given that they reproduce sexually, anyhow. Spiders do.) I discussed spider sex and the behavior that leads up to it quite extensively in my last article, so I won’t rehash it here.

But the opposite sex isn’t the only sex you have to worry about, especially if chances to mate are rare and precious. Remember those super flamboyant peacock spiders who dance erotically enough to rival Channing Tatum? Pretend you’re one of them, a male, trying to woo a vaguely interested lady. If a same-sexed competitor starts edging towards the object of your desire, how are you gonna tell him to hop off your dance floor?

You might say, “Just eat him,” and that’s probably what first comes to most spiders’ minds too, but things, alas, cannot always go so simply. I mean, on the one side, there’s this jerk trying to edge in on your one-man show, but on the other side, there’s a lady spider who is bigger and stronger and hungrier than you with a very short attention span. It pays to be delicate here.

So what do the male spiders do? They keep dancing, but turn it into aggressive dancing.

(Skip to 1:46 to see the male-male competition.)

In fact, male spiders respond to the presence of rival males in a number of different ways. Some simply increase the intensity of their courtship displays aimed towards the female- the DANCE LIKE YOU’VE NEVER DANCED BEFORE approach. Others may aim signals already present in their courtship repertoire towards the other male- the male Saitis barbipes, for example, performed leg-stretches, an ordinary component of their mating dance, towards rivals as well. Spiders of all kinds just really love waving their legs.

Competition for mates isn’t the only time that spiders of the same sex clash with one another. For example, brush-legged wolf spiders are rather territorial over their hunting grounds. Rather than engage in costly fights during every encounter, the spiders will use a variety of escalating warning signals such as leg-waving, tapping, and mock-charges to intimidate others.

The reason I bring up these types of behaviors, as simple as they are, is that their existence suggests that there are costs to being too aggressive for spiders. Perhaps a large male might simply leap upon and cannibalize a smaller one without any trouble, but if the male is close to him in size he risks becoming dinner himself. Agonistic displays help stop the struggle before it gets too dangerous. In fact, displaying works so well that some insects have evolved displays of their own to mimic their spider predators and make them back off. You see the dangers of becoming too social? Might lose your lunch!

Remember this from the last article? A moth (B. hexaselena) mimics a jumping spider predator (P. formosa). From Rota & Wagner, 2006.

By the way, male spiders are not the only ones to display agonistic behaviors; females do as well. Often it’s a warning to a male that she doesn’t want to mate with him.

(Skip to 3:00 to see the female rejecting the male; surprisingly, it involves a lot of wiggling her butt in his face.)

In fact, because of these behavioral displays, far less cannibalism- even with food-deprived animals- occurs among spiders than might be expected. (Species in which sexual cannibalism is a reproductive strategy notwithstanding.)

That’s all well and good for communication between adult spiders. But what about females and their young? Is there such a thing as a protective mama spider?

You probably won’t be too surprised to hear me say there absolutely is. Most female spiders weave protective egg sacs made of especially stiff silk to guard their eggs. These also provide young spiderlings a place to hatch and grow safely for their first few days, whether or not mom is there. But occasionally, female spiders will continue to guard their egg sacs after they construct them by chasing off potential predators.

In species such as Stegodyphus lineatus, the main threat to the egg sac comes from a surprising source: other spiders, specifically males. The males don’t eat the eggs once they get to them- rather, they detach them by dragging the egg sac to the entrance of the female’s nest and dropping it on the ground far below. The female, with no eggs, will be ready to mate again: just what the male wants. A good 8% of a S. lineatus female’s offspring are killed by opportunistic males. Unsurprisingly, females of this species can be especially aggressive towards their potential lovers.

Other spider species, like wolf spiders and nursery web spiders, circumvent this little issue by carrying their egg sacs with them wherever they go.

Wolf spider carrying her egg sac with her spinnerettes. Other species carry them in their mouths. (Source.)

Some mama spiders continue to carry their spiderlings once they emerge, until they’re strong enough to make their own way.

The Hawaiian happy face spider (yes, that is actually what it’s called) not only guards her egg sac and carries her spiderlings, but allows them to feed from her kills until they are able to fend for themselves. Individuals of this species have even been observed adopting orphaned spiderlings into their own broods!

A happy face spider (Theridion grallator) with a couple spiderlings in tow. Her butt is so happy to see you! (Source.)

Other female spiders use unfertilized eggs to feed their newborns, and others regurgitate their latest meals, allowing their babies to swarm all over their faces to suck it all up. And some even sacrifice themselves, allowing their young to make them into a nourishing snack. This endearing behavior is known as “matriphagy.”

By the way, this doesn’t occur quickly- the young feed on the bodily fluids of their dying mother for a number of hours. Delightful!

Not terribly much is known about spiderling-to-spiderling social behavior, despite the fact that in all spider species, the young spend one instar (i.e., they molt once) together in the egg sac before they emerge. So all newborn spiders have minimal-to-no aggression to their own kind: in fact, they can’t even hunt other species for a while, which is why mom might help them at the start. But generally, once they disperse from one another, they grow into solitary, aggressive hunters.

Dispersal itself may involve either skittering away through the undergrowth or the charming practice known as ‘ballooning.’ Perhaps you know it if you’ve ever read Charlotte’s Web: the spider spins a kind of reverse parachute, lets the wind catch it, and… whee!

By the way, adult spiders also balloon at times, particularly when heavy flooding drives them to migrate from their home webs. This can lead to a mass exodus of spiders to higher ground, producing some… interesting topography.

Oh, Australia. (Photo by Lukas Coch.)

It may ease your mind a little to know that the spider species known to gather together like this are completely harmless to humans. But once you get that thought out of the way, you may wonder how adult spiders, which are- as I established- generally solitary and territorial creatures can tolerate living together in such close quarters. Certainly the insect population in the area would have a sizable dent put in it.

Well, the short answer is that most don’t, at least not for very long. Sexual, territorial, and maternal behaviors are, after all, the bare minimum as far as social behaviors are concerned, and most spiders are perfectly capable with just those in their repertoire. But some do go further, and they are split into two categories: the subsocial and cooperatively social spiders. Subsocial spiders are kind of a loosely defined group, since the term subsocial itself is only loosely defined: basically, any spider species that spends part of its life in a group is considered subsocial. Under this definition, many of the protective mama spiders I spoke about above fall under that banner. But even those are at the far left of the subsocial continuum. Other subsocial spider species will hang out for long periods of time with grown-up members of their own kind, whether they be offspring or siblings or even potential mates.

One subsocial species, the orb-weaver Parawixia bistriata, has a unique system where individuals built separate, adjacent webs during the night. I do mean separate- the spiders will tussle over prime web spots and defend them. Yet as the sun rises, the growling and snapping calms down, and spiders who haven’t gotten a chance to build a web are usually allowed to snack on the remnants of other spiders’ meals. As the day gets warm, everybody huddles together a big ol’ spider love ball until the sun goes down and it’s time to spin webs again.

I feel the love crawling all over my body. (Photo source.) (By the way, the photo at the very top of this article is a P. bistriata colony in their spread-out form hanging from some Brazilian power lines.)

The flat huntsman spider (Delena cancerides) is a subsocial species that is even more cuddly than P. bistriata. Like the name implies, they are not web-spinners but rather active night hunters who retreat to a den at dawn. While young huntsman spiders are still growing, they share the nest with mom. Interestingly enough, within the nest there may be siblings from multiple broods sharing the space. Not only that, but mom isn’t the only one bringing food back for the babies: the older siblings are, too!

The flat huntsman spider may also be the only spider to have evolved a form of kin-recognition. I mentioned before that spiders like the Hawaiian happy face spider willingly accept foreign offspring into their broods- this is true of nearly all spiders that show a degree of maternal care. But not the flat huntsman spider: despite their gentle manners within the nest, these spiders will attack and eat any spider they encounter that isn’t related to them on the outside. In fact, if an unrelated spiderling is placed within the nest, it has a very high chance of getting killed (especially if it’s more than a few days old).

Yet these same cannibalistic spiders are quite considerate towards their siblings. In one rather horrifying study, huntsman spiders were placed with either a smaller sibling or a smaller non-relation without any food. The non-relations were eaten within a day, but the huntsman spiders paired with their siblings literally starved to death over six weeks rather that hurt their baby brothers or sisters. Even then, the younger ones wouldn’t feed on the bodies of the older ones!

That is pretty dang nice for a spider. A couple other subsocial spider species have been put to this test as well- and failed it.

Still, the flat huntsman spiders only live in colonies of up to three hundred individuals. Is three hundred spiders a lot of spiders? Compared to one or two, yes it is, but it is peanuts compared to the size of some other social spider colonies. We are talking tens of THOUSANDS of spiders here.

Apparently, the brownish coloration in the web is from the bodies of millions of dead mosquitoes.

These, my friends, are the the truly social spiders- the cooperatively social spiders.

Colonies this complex only occur a handful of spider species, and the fascinating thing is that they don’t appear to have a common ancestor. Cooperative social behavior evolved at least twelve different times in the spider lineage. That means that there is a very rare but very compelling set of circumstances, environmental and internal, that cause spiders to go social.

As for just what these circumstances are, the answer is naturally quite complex, because nothing tends to be simple when it comes to evolution. But let’s start with what traits most social spiders have in common.

The big one to start with is webs. All cooperatively social spiders weave them, and almost all subsocial spiders do, too- the flat huntsman spider is a big exception in many ways. In other cases, spiders from families that generally don’t spin webs to hunt have regained that ability to go social- one such example is the social lynx spider (Tapinillus sp.).

But they can’t spin just any kind of web, especially if they’re going to be cooperatively social. Remember P. bistriata, the spider that likes to form a leggy love ball? They may be cuddly when they sleep during the day, but at night they spin the flat, spiraling webs characteristic of orb weavers, and it’s a one-spider-to-a-web deal. The webs simply won’t work with multiple spiders using them to catch prey. A bunch of them would just end up triggering trap-lines all over the place and causing a great deal of confusion. Remember, web-spinning spider vision is absolutely awful, so vibrations are kind of important for them.

The webs utilized by cooperatively social spiders that hunt in groups are going to be the messy-looking, three dimensional type that are known as sheet webs or cobwebs.

Examples of web types from nine different social spider families. From Aviles, 1997.

To help differentiate the struggle vibrations of prey from the vibrations of their friends- because wouldn’t that be an unfortunate mix-up- many social spiders not only utilize three-dimensional webs but even synchronize their movements with one another in a living spider-wave. The regular vibrations from their pals are easy to differentiate from prey vibrations.

Another factor common to nearly all social spider species is their relatively small size compared to other members of their families. This may be due to a selection for paedomorphosis, or juvenile characteristics- in other words, social spiders get their friendliness by extending that tolerant “baby phase” where everybody hangs out in the egg sac and nobody tries to eat anybody else. This is the same way that the loves-everybody-he-meets dog evolved from the I-really-don’t-trust-you-and-might-bite-your-face-off wolf. Yes, I am saying that social spiders are to dogs what regular spiders are to wolves.

Indeed, most social and even subsocial spiders (aside from the flat huntsman spider I discussed above) are extraordinarily tolerant of members of their own species, related or not, and will happily fuse colonies with complete strangers. This trait is quite different from any social insect- but we’ll get to that in a second.

Paedomorphosis and three-dimensional webs may both be factors that facilitated social evolution for these spiders, but there are plenty of spiders that have shrunk in size and spun tangled webs out there that are solitary: far more than are social, in fact. We have yet to really touch on the reasons why social spiders became social.

Surprisingly, the main theory lies in where most of them live: the tropics. Those of you who know a little bit about the ecology of tropical rainforests will know that there is an extraordinary amount of niche partitioning taking place there: in other words, there is a LOT of competition from other species, so much so that everybody needs to get really specialized. Spiders, being rather generalist predators, might struggle with this, particularly if their size limits them to a certain subset of prey items. In fact, social spiders tend to be found in environments where there are much larger prey items than small ones, available year-round. But there are also a lot of different predators year-round as well. (Sometimes prey and predator are one and the same, too.)

With this particular set of pressures and opportunities, you can see how cooperative social behavior might be selected for: more individuals can work together to take down larger prey, defend young against predators, and repair webs after it rains. Actually, tropical environments facilitated cooperative social behavior in more than just spiders: it’s the proposed place of evolution for many eusocial members of the wasp, bee, and ant order, including the famous honeybee!

But enough about why social behavior evolved. I haven’t talked nearly enough about what cooperatively social spiders even do, and what sets them apart from subsocial spiders. It isn’t just colony size- some cooperatively social spiders do have colonies on the small side, with only a few hundred individuals. What sets them apart, rather, is that they do absolutely everything together, all the time. Even that cuddly, family-friendly flat huntsman spider eventually disperses from his or her nest to strike out alone. But the colony-living spiders don’t do this. Ever. They don’t disperse.

What? you may ask. How can they not ever leave home? Other colony-living invertebrates have ways to disperse- the flying ant queens and their mates, swarming and fission in honeybees, et cetera- so how do social spiders find mates that aren’t related to them?

Oh, you poor innocent thing. Maybe you’ll understand when I tell you that on average, members of a social spider colony have polymorphism levels (a measure of genetic variation) of 5-8% between them. They are inbred as all hell.

Inbreeding is actually an issue for most spiders that have a modicum of social behavior, even subsocial ones: as I said, there’s only one species known to show any sign of kin recognition, the flat huntsman spider. And they’re the one exception to this rule, with rather healthier 32-68% levels of polymorphism. Most subsocial spiders are well below that, and they do disperse.

Now, it’s not as if dispersal never ever happens among the cooperatively social spiders. For all sorts of reasons, colonies can get split up or fail, leaving a few individuals on the lam, and sometimes a few female spiders do strike out on their own for no discernible reason. As I mentioned before, other colonies will take these foundlings in with relatively little fuss, though in most cases separatists tend to succumb to the elements soon after leaving the main colony. But the difference between social spiders and pretty much any other social invertebrate- nay, social animal– is that they have no behaviors that actually trigger dispersal. If it happens, it happens more or less by accident. Otherwise, they happily mate with their cousins.

I’m putting an image of a naked mole rat here because INBREEDING. (Photo by Rochelle Buffenstein.)

There is another major difference in the way spider colonies organize themselves compared to insect colonies. Hymenopteran (ants, bees, wasps) colonies have a single breeding female known as the queen, while others like termites (usually) have a single breeding pair: the king and queen. In fact, all eusocial or pre-eusocial organisms rely on the fact that only a tiny fraction of the population breeds: this means that nonbreeders are all the offspring of a single mother, and have a genetic incentive to help care for their younger siblings.

In social spider colonies, nearly all females breed, and everybody takes care of everybody’s offspring. No discrimination involved whatsoever. This may seem rather antithetical to the very concept of kin selection- the idea that animals prefer to help others they’re closely related to- but, in fact, it is not. There is so little genetic differentiation within spider colonies that you’re probably just as related to your sister as you are your second cousin’s uncle’s mother’s grandfather’s niece by adoption.

In other words, spider colonies have so little genetic diversity that they might as well be one giant organism composed of thousands of spiders. A SUPERSPIDER.

Anelosimus eximus: WE ARE ONE BEING.

In this sense, the inbreeding ain’t so bad after all. Everybody is equally motivated to help everybody else with all the chores. Except for the males, who are tiny, but they only make up a fraction of the population and who cares about them.

And if you thought a solitary hunting spider spelled doom for an insect, well…

Imagine having your innards sucked out through a hundred tiny straws. 😉

With the power of sheer numbers, these spiders- which are generally about 5 millimeters long- have reportedly taken down prey as large as rats. People have found small mammal skulls tangled in their webs.

…Luckily, they aren’t dangerous to humans at all. YET.

(Ok, but seriously, they never will be, no need for more Bad Spider Press.)

The point is that their numbers allow them to take on nearly any insect that stumbles into their web- which is good, because if they were only dealing with tiny ones, they wouldn’t be able to keep feeding their numbers. They need that very specific tropical environment where there are lots of big bugs all the time to keep themselves going.

And it isn’t just a free-for-all on the prey items, either. Even if prey is caught in a web, they may still be able to struggle their way out of it. This means that hunting spiders are going to have to be fast and efficient at subduing the insect if they don’t want to be left with empty stomachs and a gaping hole in the web.

Studies on both social and subsocial spiders have found a unique degree of coordination in their responses to struggling prey. Young Amaurobius ferox spiders from the same brood stay together for a while after they eat their mother (it’s ok, it’s how she wanted to go) and build sticky little webs of silk stretched over stones or crevices. When an insect gets caught, the spiderlings rush them in organized waves. The first to arrive grab the prey by its antennae and legs, pulling them in opposite directions a la that one medieval torture device. With the prey thus immobilized, the next wave can inject venom into its tender abdomen. Using this strategy, teeny tiny spiders can take down prey over ten times their size.

Black ovals represent spiders on this figure, and each also corresponds to a degree of cricket panic. From Kim et al., 2005.

So now we know social spiders are pretty good at hunting together. But this brings up another question: how do they divide up their labor? How do the spiders decide who gets to do the hunting, who gets to do the web-spinning, and who gets to do the child-rearing?

You might think, as with breeding, everybody does everything. But there is a reason that other colony species divide themselves up into castes like workers and soldiers: it’s more efficient. If Jenny Bee is nursing a grub and then hears the call to go out and get nectar, it’s not super great if she immediately drops the grub on the floor and flies outta there. You gotta… you gotta prioritize, Jenny Bee.

Social insects have several ways of dividing themselves into castes- it can be by birth phenotype, as in ants, or by age, as in honeybees, for example. How about social spiders- do they divide themselves into castes? Well… as with everything else, social spiders are a little weird in this respect.

They do sort of divide up the labor, though it’s arguable whether or not you could claim they have castes. But there are spiders that usually hunt, spiders that usually nurse the young, spiders that repair the webs, and so on. As for the great determiner of who does what- there really isn’t one.

Not one as ironclad as birth or age, anyway. Indeed, a social spider may actually switch jobs during her life several times. The real determining factor of who does what actually seems to be… wait for it… personality.

Yes, spider personality: that is the official theory.

I imagine you have your head in your hands right now, wondering what the hell a spider personality would even look like. How does one determine where a spider falls on the Briggs-Meyers personality test? Do they have extremely tiny spider questionnaires? What differentiates an ISTJ spider from an ENFP spider?

Alright, kidding aside, this is actually one of the most fascinating things about social spiders. Researchers have identified multiple personality types in social spiders so far, specifically in Anelosimus species. In one experiment, researchers classified the spiders as either “bold” or “shy.” They determined this by blowing on spiders so that they retracted their legs and pretended to be pebbles. The spiders who un-pebbled the quickest were the bold ones, while the ones that remained curled up were the shy ones.

A separate experiment used a different test to determine two other spider personality traits: they put two spiders in a box, and considered ones that stayed close together “docile” and ones that moved further apart “aggressive.” As you might imagine, the spiders that had higher docile scores tended to be the ones taking care of the babies, while the aggressive ones tended to be the hunters and colony defenders.

What about bold and shy spiders from the previous experiment? Well, the bolder ones were definitely more involved in defense and hunting, but really sucked at keeping babies alive. They didn’t test the shy ones for comparison, though.

One more fun fact about that particular experiment- the researchers used a pink vibrator- yes, THAT kind of pink vibrator- to shake the spiderwebs. Because it had reproducible vibrations. Apparently they bought it just for the study.

This whole personality bit is especially interesting because of how low the genetic diversity is within the spider colonies; furthermore, there aren’t any obvious physical traits associated with each personality type: for instance bold, aggressive spiders that like to hunt may be smaller than shy, docile ones that like to nurse babies. More importantly, having a particular personality trait didn’t mean a spider was ‘locked in’ to a specific task: it just predicted what they were most likely to do. Could spider personality be a factor of the environment, rather than just an encoded genetic trait? In other words, is it possible for spider personality to be shaped and changed over time?

Some research says yes, at least partially. While the traits do appear to be somewhat heritable, the amount that they are expressed actually depends on the social group that the spider is with.

This was demonstrated in yet another study that involved disturbing poor defenseless spiders. In this case, spiders were first assessed for bold/shy personality traits via the pebble test. Then the researchers ruined each colony’s nests like the jerks that they are. In addition, they shuffled around the members of some colonies so that they were hanging out with a completely new bunch of gals.

The result? When spiders were with others they were familiar with, their personalities were strong and consistent: bold spiders were BOLD, and shy spiders were SHY. But in an unfamiliar group, the differences in personality seemed to fade away, and spiders behaved pretty uniformly. It took them a long time to regain a degree of personality difference, and when they did, not everyone showed the same traits that they had before.

You may have experienced this phenomenon yourself, when figuring out how to interact with a brand new group of people- your personality is somewhat ‘muted’ until you figure out just where you fit in. It’s called social niche specialization, and it’s been found in other animals, not just humans and spiders. As the theory goes, each member of a social group gradually takes on a specific social “role,” whether it’s being the boisterous confident one or the quiet thoughtful one- and the roles may be different for the same individual depending on which people they’re around. Strange as it seems, spiders too take on different roles based on group composition.

Nobody EVER wants to work with Sheila here (Cebrennus rechenbergi, and yes they really do this).

It is a bit ironic to have individual personality be a defining factor in species that depends on its genetic similarity for existence, isn’t it? I suppose that just goes to show you that genes don’t make (all of) the man. Er, the spider. The spider-man.

But coming back to that whole inbreeding thing: it may have occurred to you earlier than this that inbreeding can be, ehm, problematic for the survival of the species. While it is true that many species seem to be protected from the deleterious effects of inbreeding depression- and social spiders are- there are other big issues with letting everybody get too genetically similar to one another. Namely, if everybody’s too similar, it might only take one bad disease, parasite, predator, or natural disaster to take them out. Things get more dangerous when you consider the fact that dispersal in social spiders is rather limited, AND they only thrive in very specific environmental conditions, as I discussed earlier.

Even more damning is the fact that though cooperative social behavior has evolved multiple times, separately, in spiders, these speciations all occurred relatively recently, leaving the social spiders at the very tips of the spider family tree. What this means is that there’s no evidence for a social spider species that lasted for more than a few million years; it could be that many evolved and died out that we simply don’t know about, leaving no evidence for their earlier existence, and no descendents. And that’s another problem with limited genetic diversity- it is very hard to split into new species when everybody ends up with pretty much the same genes.

So cooperatively social spiders, despite their seemingly egalitarian and efficient societies, may all be one nasty accident away from going extinct entirely. They may be at what is known as “evolutionary dead ends.”

But there is a little hope for social spiders. The fact that they have no regular means of dispersal is actually a boon in this sense. The populations of social spiders generally rise and fall in chaotic manners, and fission and dispersing events seem to happen by chance. Irregular population patterns are actually protective against catastrophic events like disease or environmental disturbance because the spiders don’t depend on specific cues to expand or split their colonies.

I myself would love to have these amazing creatures around for a few million years more, because they are crazy weird and I can’t learn enough about them. But even if they are functionally dead branches of the evolutionary tree, the rest of spider-kind isn’t going anywhere, and history suggests that when conditions are right, lineages of social spiders will evolve again.

I hope you’ve enjoyed Spider Behavior Month(s), and the next time you see a spider, maybe you’ll have a better appreciation for the truly amazing critters that they are. Happy web-spinning, Spiderfriends!

This is a large and beautiful wolf spider friend I made today. Completely harmless, and happy to go back outside where there were more things to eat! Be good to spiders, folks.